Resonant actuator systems are used in a variety of different applications, such as to move optics within cameras by way of example only. Examples of resonant actuator systems may be found in U.S. Pat. No. 6,940,209, titled, “Ultrasonic Lead Screw Motor”; U.S. Pat. No. 7,339,306, titled, “Mechanism Comprised of Ultrasonic Lead Screw Motor”; U.S. Pat. No. 7,170,214, titled, “Mechanism Comprised of Ultrasonic Lead Screw Motor”; and U.S. Pat. No. 7,309,943, titled, “Mechanism Comprised of Ultrasonic Lead Screw Motor,” which are hereby incorporated herein by reference in their entireties. In these different applications, control over the velocity of operation while at the same time reducing power consumed for extended battery life and component life of the resonant actuator systems often is required.
To maximize the performance of a reduced voltage, resonant actuator, prior systems often have used a full bridge driver to double the available supply voltage applied to the resonant actuator. However, the use of a full bridge driver can make speed regulation as well as management of power consumption more difficult.
More specifically, when switching capacitive loads with a full bridge driver switching sequence, the power dissipation in the switches is provided by equation (1):
                    P        =                              2            ·                                          C                ·                                                      (                                          2                      ·                                              V                        DD                                                              )                                    2                                            2                        ·            f                    =                      4            ·            C            ·                          V              DD              2                        ·            f                                              (        1        )            since the voltage across the switches is twice the supply voltage VDD. The transitions for this switching sequence are monotonic and continuous back and forth between the positive and negative limits of the available supply voltage.
As discussed earlier, the use of such a continuous full bridge driver sequence using a full bridge driver circuitry can make speed regulation as well as management of power consumption more difficult. Modulation of pulse width has been used to provide effective regulation of output speed, but unfortunately saves little in power consumption because the switching frequency of the full bridge sequence does not change.